Converter topologies and control

ABSTRACT

Systems, methods, and articles of manufacture are provided wherein inverter topologies and inverter control employ primary and secondary windings with a half-bridge circuit and an unfolding bridge circuit positioned between the second winding and an AC grid. Certain topologies may employ control circuits for controlling the bridges suitable for a phase angle of the AC grid.

CROSS-REFERENCE TO COMMONLY-OWNED CO-PENDING U.S. PATENT APPLICATIONS

The present application is related to commonly-owned co-pending U.S.patent application Ser. No. 15/080,110 entitled “DC-TO-AC INVERTERTOPOLOGIES” by Fernando Rodriguez, Hengsi Qin and Patrick Chapman, whichwas filed on Mar. ______, 2016 and claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 62/138,184,entitled “DC-TO-AC INVERTER TOPOLOGIES” by Patrick Chapman, which wasfiled on Mar. 25, 2015. The entirety of both applications are herebyfully incorporated, into this application, by reference.

TECHNICAL FIELD

The present disclosure relates, generally, to power converters forconverting direct current (DC) power to alternating current (AC) powerand, more particularly, to converter topologies and control techniques.

BACKGROUND

Power inverters convert a DC power to an AC power. For example, somepower inverters are configured to convert a DC power to an AC powersuitable for supplying energy to an AC grid and, in some cases, an ACload that may or may not be coupled to the AC grid. One particularapplication for such power inverters is the conversion of DC powergenerated by an alternative energy source, such as photovoltaic cells(“PV cells” or “solar cells”); fuel cells; DC wind turbines; DC waterturbines; and other DC power sources, to a single-phase AC power fordelivery to the AC grid at the grid frequency. The amount of power thatcan be delivered by certain alternative energy sources, such as PVcells, may vary in magnitude over time due to temporal variations inoperating conditions. For example, the output of a typical PV cell willvary as a function of variations in sunlight intensity, angle ofincidence of sunlight, ambient temperature and other factors.

In a typical photovoltaic power system, an inverter may be associatedwith one or more solar cell panels. For example, some systems includestrings of solar cell panels that deliver a relatively high, combinedvoltage (e.g., nominal 450 V) to a single, large inverter.Alternatively, in other systems such as a distributed photovoltaic powersystem, an inverter may be associated with each solar cell panel. Insuch systems, the solar cell panels are typically small with relativelylow voltage (e.g., 25 V). The inverter may be placed in close proximityto the associated solar cell panel to increase the conversion efficiencyof the overall system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram as may be employed in embodimentsof a system for converting DC power to AC power;

FIG. 2 is a simplified block diagram of an AC photovoltaic module of thesystem of FIG. 1;

FIG. 3 is a simplified block diagram of an inverter of the system ofFIG. 1 or as may otherwise be employed in embodiments;

FIG. 4 is a simplified electrical schematic of an embodiment of theinverter of FIG. 3 or as may otherwise be employed in embodiments;

FIG. 5 is a simplified electrical schematic of the AC-AC converter ofFIG. 3 or as may otherwise be employed in embodiments;

FIG. 6 is a simplified electrical schematic of available topologies ofthe AC-AC converter of FIG. 3 or as may otherwise be employed inembodiments;

FIG. 7 illustrates a module configured for estimating the grid phaseangle from grid voltage as may be employed in embodiments;

FIG. 8A is a simplified electrical schematic identifying the fourswitches in an unfolding bridge that may be used in the inverter of thesystem of FIG. 1 as well as other embodiments;

FIG. 8B is a representative plot of the grid voltage relative to thegrid phase angle illustrating the timing of pairs of switches as theyare turned on and off as may be employed in embodiments; and

FIG. 9 is a simplified flow diagram of an embodiment of a method forcontrolling the inverter of FIG. 1 or may otherwise be employed inembodiments.

DETAILED DESCRIPTION

While the concepts of the present disclosure are susceptible to variousmodifications and alternative forms, specific exemplary embodimentsthereof have been shown by way of example in the drawings and willherein be described in detail. It should be understood, however, thatthere is no intent to limit the concepts of the present disclosure tothe particular forms disclosed, but on the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

References in the specification to “one embodiment”, “an embodiment”,“an example embodiment”, etc., indicate that the embodiment describedmay include a particular feature, structure, or characteristic, butevery embodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to effect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

Some embodiments of the disclosure, or portions thereof, may beimplemented in hardware, firmware, software, or any combination thereof.Embodiments of the disclosure may also be implemented as instructionsstored on a tangible, machine-readable storage medium, which may be readand executed by one or more processors. A machine-readable medium mayinclude any mechanism for storing or transmitting information in a formreadable by a machine (e.g., a computing device). For example, amachine-readable medium may include read only memory (ROM); randomaccess memory (RAM); magnetic disk storage media; optical storage media;flash memory devices; and others.

This specification includes references to “one embodiment” or “anembodiment.” The appearances of the phrases “in one embodiment” or “inan embodiment” do not necessarily refer to the same embodiment.Particular features, structures, or characteristics may be combined inany suitable manner consistent with this disclosure.

Terminology. The following paragraphs provide definitions and/or contextfor terms found in this disclosure (including the appended claims):

“Comprising.” This term is open-ended. As used in the appended claims,this term does not foreclose additional structure or steps.

“Configured To.” Various units or components may be described or claimedas “configured to” perform a task or tasks. In such contexts,“configured to” is used to connote structure by indicating that theunits/components include structure that performs those task or tasksduring operation. As such, the unit/component can be said to beconfigured to perform the task even when the specified unit/component isnot currently operational (e.g., is not on/active). Reciting that aunit/circuit/component is “configured to” perform one or more tasks isexpressly intended not to invoke 35 U.S.C. § 112, sixth paragraph, forthat unit/component.

“First,” “Second,” etc. As used herein, these terms are used as labelsfor nouns that they precede, and do not imply any type of ordering(e.g., spatial, temporal, logical, etc.).

“Based On.” As used herein, this term is used to describe one or morefactors that affect a determination. This term does not forecloseadditional factors that may affect a determination. That is, adetermination may be solely based on those factors or based, at least inpart, on those factors. Consider the phrase “determine A based on B.”While B may be a factor that affects the determination of A, such aphrase does not foreclose the determination of A from also being basedon C. In other instances, A may be determined based solely on B.

“Coupled”—The following description refers to elements or nodes orfeatures being “coupled” together. As used herein, unless expresslystated otherwise, “coupled” means that one element/node/feature isdirectly or indirectly joined to (or directly or indirectly communicateswith) another element/node/feature, and not necessarily mechanically.

“Inhibit”—As used herein, inhibit is used to describe a reducing orminimizing effect. When a component or feature is described asinhibiting an action, motion, or condition it may completely prevent theresult or outcome or future state completely. Additionally, “inhibit”can also refer to a reduction or lessening of the outcome, performance,and/or effect which might otherwise occur. Accordingly, when acomponent, element, or feature is referred to as inhibiting a result orstate, it need not completely prevent or eliminate the result or state.

In addition, certain terminology may also be used in the followingdescription for the purpose of reference only, and thus are not intendedto be limiting. For example, terms such as “upper”, “lower”, “above”,and “below” refer to directions in the drawings to which reference ismade. Terms such as “front”, “back”, “rear”, “side”, “outboard”, and“inboard” describe the orientation and/or location of portions of thecomponent within a consistent but arbitrary frame of reference which ismade clear by reference to the text and the associated drawingsdescribing the component under discussion. Such terminology may includethe words specifically mentioned above, derivatives thereof, and wordsof similar import.

In this description, numerous specific details are set forth, such asspecific operations, in order to provide a thorough understanding ofembodiments of the present disclosure. It will be apparent to oneskilled in the art that embodiments of the present disclosure may bepracticed without these specific details. In other instances, well-knowntechniques are not described in detail in order to not unnecessarilyobscure embodiments of the present disclosure.

Embodiments may comprise a multi-port inverter for converting an inputdirect current (DC) waveform from a DC source to an output alternatingcurrent (AC) waveform for delivery to an AC grid may include atransformer that includes a first winding and at least a second winding.The inverter may further include a DC-AC inverter electrically coupledto the first winding of the transformer, an AC-AC converter electricallycoupled to the second winding of the transformer, and an active filterelectrically coupled to a second winding of the transformer. The DC-ACinverter may be adapted to convert the input DC waveform to an ACwaveform delivered to the transformer at the first winding. The AC-ACconverter may be adapted to convert an AC waveform received at thesecond winding of the transformer to the output AC waveform having agrid frequency of the AC grid. The active filter may be adapted to sinkand source power with one or more energy storage devices based on amismatch in power between the DC source and the AC grid.

The AC-AC converter may include a first set of electrical switcheselectrically coupled to a first terminal of the second winding of thetransformer, a capacitor divider electrically coupled with the first setof electrical switches and to a second terminal of the second winding ofthe transformer, a second set of electrical switches electricallycoupled to the AC grid, a first capacitor electrically coupled acrossthe first set of electrical switches, and a resistor electricallycoupled between the first capacitor and the second set of electricalswitches.

Embodiments may also comprise a multi-port inverter for converting aninput direct current (DC) waveform from a DC source to an outputalternating current (AC) waveform for delivery to an AC grid. Thistopology may include an AC-AC converter electrically coupled through atransformer to a DC-AC inverter electrically coupled to the DC sourceadapted to convert an AC waveform received from the transformer tooutput the AC waveform having a grid frequency of the AC grid. The AC-ACconverter may include a half-bridge circuit electrically coupled to thetransformer and an unfolding bridge circuit electrically coupled betweenthe half-bridge circuit and the AC grid. The inverter may also include acontroller having a processor and a memory wherein the controller isadapted to control the switching cycles of electrical switches of theunfolding bridge circuit. These cycles may trigger the switches suchthat when a voltage across the AC grid is substantially positive duringa first period, a first set of electrical switches is on and a secondset of electrical switches is off. The cycle may also trigger theswitches such that when the voltage across the AC grid is substantiallynegative during a second period, the first set of electrical switches isoff and the second set of electrical switches is on. Still further, thecycle may also include a third period comprising a blanking time periodbetween the first and second periods such that when the voltage acrossthe AC grid is approximately zero, the first and second sets ofelectrical switches are off.

Embodiments may also further comprise a multi-port inverter topology forconverting an input direct current (DC) waveform from a DC source to anoutput alternating current (AC) waveform for delivery to an AC grid.This topology may include a transformer that includes a first windingand at least a second winding, a DC-AC inverter electrically coupled tothe first winding of the transformer adapted to convert the input DCwaveform to an AC waveform delivered to the transformer at the firstwinding, an AC-AC converter electrically coupled to the second windingof the transformer and adapted to convert the AC waveform received atthe second winding of the transformer to the output AC waveform having agrid frequency of the AC grid, an active filter electrically coupled tothe at least second winding of the transformer wherein the active filteris adapted to sink and source power with one or more energy storagedevices based on a mismatch in power between the DC source and the ACgrid, and a controller, comprising a phase locked loop (PLL)electrically coupled to receive an AC voltage from the AC grid andhaving an output signal comprising an estimate of the phase angle of theAC voltage, wherein the controller, in response to the estimated phaseangle of the AC voltage, controls the switching cycles of a plurality ofelectrical switches of the AC-AC converter. The AC-AC converter mayinclude a half-bridge circuit electrically coupled to the first windingof the transformer and an unfolding bridge circuit electrically coupledbetween the half-bridge circuit and the AC grid.

In the aforementioned commonly-owned application, U.S. Ser. No.15/080,110, a number of DC-AC inverter topologies are disclosed. Generaltopologies therein comprise a multi-winding transformer, a DC-ACinverter electrically coupled between a DC source and winding of thetransformer, an active filter electrically coupled to a winding of thetransformer, an AC-AC cycloconverter, electrically coupled between awinding of the transformer and an AC grid, and an inverter controllerelectrically coupled to the DC-AC inverter, the active filter, and thecycloconverter. In embodiments described and illustrated, thecycloconverter comprises a resonant tank circuit and a circuit thatresembles a half-bridge circuit. The half-bridge circuit uses twofull-blocking switches that allow the voltage source, the AC grid, to bebi-polar. The full-blocking circuit of the cycloconverter comprises twocommon source MOSFETS, which doubles the conduction losses compared witha half-bridge circuit.

In embodiments of the present invention, the cycloconverter is replacedby an AC-AC converter comprising a half-bridge circuit and an unfoldingbridge circuit. Simulations have indicated that such a topology performsas expected and generates power at a unity power factor as well asleading and lagging reactive power. Performance of the electromagneticinterference (EMI) filter in the topologies of the present inventionsmay be comparable to that of the EMI filter in the cycloconverter.

Compared to the cycloconverter, conduction losses may be reduced inembodiments of the AC-AC converter of the present invention. Forexample, for a given operating condition, both the half bridge of thepresent invention and the cycloconverter switch at high frequency canprocess current with a high RMS magnitude. However, given that thecycloconverter has four switches while the half-bridge has two switches,conduction losses may be reduced in half or by another amount.

Additionally, the gate driver power supply for the AC-AC converter ofthe present invention may be less complex than in topologies employing acycloconverter. For example, cycloconverter topologies comprise twopairs of full blocking switches that require two isolated gate driverpower supplies. Comparatively, in embodiments of the present invention,a half-bridge plus unfolding bridge circuitry may be provided and mayrequire only a single non-isolated gate driver power supply.

Circuits employing the half-bridge and unfolding bridge topology ofembodiments may also have reduced losses. These losses may be derivedfrom synchronous rectification of the AC grid voltage performed by theunfolding bridge. This synchronous rectification may employ switching at60 Hz near zero-voltage transitions. Therefore, switching loses are verylow. Conduction losses may also be reduced in embodiments as currentthrough the unfolding bridge is the average output current of the highfrequency resonant half-bridge, which results in reduced conductionlosses.

In embodiments, replacing the cycloconverter with the topology of theembodiments of the present invention may be substantially cost neutral.The cycloconverter requires four high performing switches, two isolatedgate drivers, and two isolated gate driver power supplies.Comparatively, the AC-AC converter of the present invention comprisessix switches, the half-bridge comprises two high performing switches andthe unfolding bridge comprises four low to average performing switches.This AC-AC converter can require three gate drivers; however, none needto have isolation and can, therefor, all share one gate driver powersupply.

Referring to FIG. 1, a system 100 for supplying alternating current(hereinafter “AC”) power to an AC grid 102 at a grid frequency includesa direct current (hereinafter “DC”) source 104 and an inverter 106. TheDC source 104 may be embodied as any type of DC source configured togenerate or produce a DC power, which is supplied to the inverter 106.For example, the DC power may be embodied as a photovoltaic solar cellor array, a fuel cell, a wind turbine configured to generate a DC power(e.g., via a rectifying circuit), a water turbine configured to generatea DC power, or other unipolar power source.

The inverter 106 is electrically connected to the DC source 104 andconfigured to convert a DC waveform generated by the DC source 104 to anAC waveform suitable for delivery to the AC grid 102 and, in someembodiments, loads coupled to the AC grid 102. The AC grid 102 may beembodied as, for example, a utility power grid that supplies utility ACpower to residential and commercial users. Such utility power grids maybe characterized as having an essentially sinusoidal bipolar voltage ata fixed grid frequency (e.g., f=ω)/2π=50 Hz or 60 Hz).

As discussed above, in some embodiments, the DC source 104 may beembodied as one or more photovoltaic cells. In such embodiments, the DCsource 104 and the inverter 106 may be associated with each other toembody an AC photovoltaic module (ACPV) 200, as illustrated in FIG. 2.The ACPV 200 includes a DC photovoltaic module (DCPV) 202, whichoperates as the DC source 104, electrically coupled to the inverter 106.The DCPV 202 includes one or more photovoltaic cells and is configuredto deliver a DC waveform to the inverter 106 in response to receiving anamount of sunlight. The DC power delivered by the ACPV 200 is a functionof environmental variables, such as, e.g., sunlight intensity, sunlightangle of incidence and temperature. In some embodiments, the inverter106 is positioned in a housing of the ACPV 200. Alternatively, theinverter 106 may include its own housing secured to the housing of theACPV 200. Additionally, in some embodiments, the inverter 106 isseparate from the housing, but located near the DCPV 202. As discussedabove, the inverter 106 is configured to convert the DC power receivedfrom the DCPV 202 to an AC power suitable for delivery to the AC grid102 at the grid frequency. It should be appreciated that multiple ACPVs200 may be used to form a solar array with each ACPV 200 having adedicated inverter 106.

Referring now to FIG. 3, in some embodiments, the inverter 106 includesa DC-AC inverter 300, a transformer 302, a AC-AC converter 304, and anactive filter 306. Depending on the particular embodiment, thetransformer 302 may be embodied as a three-winding transformer thatincludes a first winding, a second winding, and a third winding or atwo-winding transformer that includes a first winding and a secondwinding (see, for example, FIGS. 4-5). Although the transformer 302 maybe described herein as a two-winding transformer or a three-windingtransformer, it should be appreciated that such transformers may includemore than two or three windings, respectively, in some embodiments. Forexample, in various embodiments, a three-winding transformer may includethree windings, four windings, five windings, or a greater number ofwindings.

The DC-AC inverter 300 is electrically coupled to the first winding (notshown) of the transformer 302 and is electrically couplable to the DCsource 104. As shown in FIG. 3, the DC-AC inverter 300 includes a DC-ACinverter circuit 310 and, in some embodiments, may include a resonanttank circuit 312 or a portion thereof. The DC-AC inverter circuit 310 isadapted to convert an input DC waveform from the DC source 14 to an ACwaveform delivered to the transformer 302 at the first winding. In someembodiments, the resonant tank circuit 312 includes a capacitor and aninductor. It should be appreciated that, in some embodiments, theresonant tank circuit 312 may be formed by one or more discretecapacitors (e.g., a capacitor divider) and a leakage inductance of thetransformer 302 (e.g., in half-bridge inverter embodiments).

The AC-AC converter 304 is electrically coupled to the second winding(not shown) of the transformer 302 and electrically couplable to the ACgrid 102. As shown in FIG. 3, the AC-AC converter 304 includes ahalf-bridge circuit 314 and an unfolding bridge circuit 316. The AC-ACconverter circuit 304 is adapted to convert an AC waveform received atthe second winding of the transformer 302 to the output AC waveformdelivered to the AC grid 102 and having the same frequency as a waveformof the AC grid 102 (i.e., the grid frequency). That is, the AC-ACconverter 304 is configured to convert an input AC waveform to an outputAC waveform having a frequency that is different from the input ACwaveform.

Depending on the particular embodiment, the active filter 306 may becoupled to the first winding, the second winding, or the third winding(not shown) of the transformer 302. For example, in embodiments in whichthe transformer 302 is embodied as a three-winding transformer, theactive filter 306 may be electrically coupled to the third winding ofthe transformer 302, whereas in embodiments in which the transformer 302is embodied as a two-winding transformer, the active filter 306 may beelectrically coupled to the first winding or, as illustrated in FIG. 4,the second winding of the transformer 302. The active filter 306 isadapted to sink and source power with one or more energy storage devices320 of the active filter 306 and using a DC-AC inverter circuit 318based on a mismatch in power (e.g., an instantaneous mismatch in power)between the DC source 104 and the AC grid 102. That is, the activefilter 306 supplies power from or absorbs power with the one or moreenergy storage devices 320 based on the mismatch in power.

For example, it should be appreciated that the DC source 104 delivers arelatively constant power to the DC-AC inverter 300. However, the ACgrid 102 has a relatively sinusoidal power that fluctuates (e.g.,between zero and peak power). When the power of the AC grid 102 is zero,the power delivered to the AC grid 102 should also be zero; accordingly,the constant power delivered by the DC source 104 is supplied to the oneor more energy storage devices 320 of the active filter 306. However,when the AC grid 102 is at peak power, the power of the AC grid 102 isgenerally twice that of the input power from the DC source 104; as such,all of the power from the DC source 104 is delivered to the AC grid 102and the other half of the power is supplied from the one or more energystorage devices 320 of the active filter 306. In some embodiments, theone or more energy storage devices 320 are embodied as one or morecapacitors; however, the energy storage devices 320 may be embodied asother devices in other embodiments.

The inverter 106 also includes an inverter controller 308, whichcontrols the operation of the DC-AC inverter 300, the AC-AC converter304, and the active filter 306. Although the inverter controller 308 isillustratively embodied as a single controller in the embodiment of FIG.3, the inverter controller 308 may be embodied as multiple separatecontrollers in other embodiments. For example, in some embodiments, theinverter 106 may include an input controller to control the operation ofthe DC-AC inverter 300, an output controller to control the operation ofthe AC-AC converter 304, and/or a filter controller to control theoperation of the active filter 306. In such embodiments, each of thecontrollers may be galvanically isolated from one another.

As discussed above, the inverter controller 308 is electrically coupledto and adapted to control operation of the DC-AC inverter 300, the AC-ACconverter 304, and the active filter 306. To do so, the invertercontroller 308 may provide a plurality of switching and/or controlsignals to various circuits of the DC-AC inverter 300, the AC-ACconverter 304, and the active filter 306. For example, in someembodiments, the inverter controller 308 controls the operation of theDC-AC inverter 300 based on a global maximum power point tracking(“MPPT”) method. As shown in FIG. 3, the illustrative invertercontroller 308 utilizes an algorithm to control various switches of theinverter 106. To do so, the inverter controller 308 may provide aplurality of switching and/or control signals to various circuits of theinverter 106. In embodiments, such signals may be repeated duty cyclesignals, e.g. 50% duty cycle signals, for each of the three ports of theinverter 106, with a small blanking time and appropriate phases shiftsbetween duty cycles at each port. It should be appreciated that, in someembodiments, the inverter controller 308 is adapted to control switchingcycles of the various electrical switches of the DC-AC inverter 300, theAC-AC converter 304, and/or the active filter 306 using zero-voltageswitching techniques.

The inverter controller 308 may include a processor 324 and a memory326, both of which may be integrated into a single integrated circuit oras separate integrated circuits connected via wires on a printed circuitboard. The processor 324 may execute instructions stored on the memory326 and cause the inverter controller 308 to perform various actions tocontrol the DC-AC inverter 300, the AC-AC converter 304, and/or theactive filter 306. The memory 326 may be any of a number of knowntangible storage mediums (e.g., RAM, DRAM, SRAM, ROM, EEPROM, Flashmemory, etc.).

Additionally, in some embodiments, the inverter 106 may include circuitsnot shown herein for clarity of the description. For example, theinverter 106 may include communication circuitry, which may becommunicatively coupled to the inverter controller 308 or may beincorporated therein. In such embodiments, the inverter controller 308may utilize the communication circuitry to communicate with remotedevices, such as remote controllers or servers. For example, dependingon the particular embodiment, the communication circuitry may beconfigured to communicate with remote devices over an AC power line,such as the AC power line interconnects coupled to the output of theAC-AC converter 304, or using other communication technologies and/orprotocols. For example, in some embodiments, the communication circuitrymay be embodied as a wireless or wired communication circuit configuredto communicate with remote devices utilizing one or more wireless orwired communication technologies and/or protocols such as Wi-Fi™,Zigbee®, ModBus®, WiMAX, Wireless USB, Bluetooth®, TCP/IP, USB, CAN-bus,HomePNA™, and/or other wired or wireless communication technology and/orprotocol. Further, in some embodiments, the inverter 106 may include aninput filter electrically coupled (e.g., in series) with the DC source104 and/or an output filter electrically coupled (e.g., in series) withthe AC grid 102.

Referring now to FIG. 4, a multi-port resonant converter topology inwhich the inverter 106 is embodied as a three-port inverter 500, andincludes a two-winding transformer 302, is shown. The illustrativeinverter 500 of FIG. 4 includes a set of full and/or half bridgeconverter circuits 502, 506, a set of impedances 508, 510, 512, thehalf-bridge circuit 314, and the unfolding bridge circuit 316. As shown,in the illustrative embodiment, the converter circuit 502 and theimpedance 508 form the DC-AC inverter 300, the half-bridge circuit 314,the impedance 510, and the unfolding bridge 316 form the AC-AC converter304, and the converter circuit 506, the impedance 512, and the energystorage device 320 form the active filter 306. As shown and describedabove, the DC-AC inverter 300 is electrically coupled to the firstwinding 414 of the transformer 302 and the AC-AC converter 304 and theactive filter 306 are electrically coupled to the second winding 416 ofthe transformer 302. It should be appreciated that two-windingtransformer 302 topologies may reduce the complexity and, therefore,cost associated with manufacturing the transformer 302 compared tothree-winding transformer topologies. In other embodiments, the DC-ACinverter 300 and the active filter 306 may be electrically coupled tothe first winding 414 and the AC-AC converter 304 may be electricallycoupled to the second winding 416. In embodiments, the transformer mayhave three windings with the DC-AC inverter 300 electrically coupled tothe first winding, the AC-AC converter 304 electrically coupled to thesecond winding, the active filter 306 electrically coupled to the thirdwinding.

The converter circuit 502 may be embodied as the DC-AC inverter circuit310 and, depending on the particular embodiment, may be embodied as ahalf-bridge inverter circuit or a full-bridge inverter circuit.Similarly, the converter circuit 506 is embodied as the DC-AC invertercircuit 318, which depending on the particular embodiment, may beembodied as a half-bridge inverter circuit or a full-bridge invertercircuit. The illustrative AC-AC converter circuit 304 is embodied as thehalf-bridge circuit 314 and unfolding bridge circuit 316. The impedance508 may be representative of leakage inductances from the two-windingtransformer 302. The impedance 512 may be representative of theimpedance of a trace on the printed circuit board on which the inverter106 is assembled. The impedance 510 may comprise a discrete component ofthe AC-AC converter 304.

As shown in FIG. 4, the half-bridge 314 of the AC-AC converter 304includes electrical switches 520, 522, a capacitor divider comprisingcapacitors 524, 526, and an inductor 510. The unfolding bridge 316 ofthe AC-AC converter 304 includes electrical switches 530, 532, 534, 536.The AC-AC converter 304 also includes a capacitor 540 and a resistor542. The resistor 542 senses the current of the AC grid to permit thecurrent to be regulated.

More specifically, first terminals of the electrical switches 520, 522are electrically coupled to one another and to a first terminal of theinductor 510. The second terminal of the inductor 510 is electricallycoupled to the second winding 416 of the transformer 302. A firstterminal of the capacitor 524 is electrically coupled to a firstterminal of the capacitor 526 and to the second terminal of the secondwinding 416 of the transformer 302. The second terminal of the capacitor524 is electrically coupled to the second terminal of the electricalswitch 520 and the second terminal of the capacitor 526 is electricallycoupled to the second terminal of the electrical switch 522.

The unfolding bridge 316 of the AC-AC converter 304 is electricallycoupled to the AC grid 102. More specifically, first terminals of eachof a first pair of the electrical switches 530, 532 are electricallycoupled to each other, to the second terminal of the switch 520 and tothe second terminal of the capacitor 524. Similarly, first terminals ofeach of a second pair of the electrical switches 534, 536 areelectrically coupled to each other, to the second terminal of the switch522 and to a first terminal of the resistor 542. Second terminals of theswitches 530, 534 are electrically coupled to each other and to one sideof the AC grid 102. Second terminals of the electrical switches 532, 536are electrically coupled to each other and to the other side of the ACgrid 102.

A first terminal of the capacitor 540 is electrically coupled to thesecond terminal of the switch 520. The second terminal of the capacitor540 is electrically coupled to a second terminal of the resistor 542 andto the second terminal of the switch 522.

In embodiments, the AC-AC converter 304 may also include an EMI filter.The EMI filter may include the two inductors and the common modeinductor, collectively identified in FIGS. 5 and 6 by the dashed box 552and electrically coupled between the unfolding bridge 316 and the ACgrid 102, and the capacitor 540, electrically coupled between thehalf-bridge 314 and the unfolding bridge 316. In one embodiment,illustrated in FIG. 5, the EMI filter 550A is comprises additionalcomponents electrically coupled between the unfolding bridge 316 and theAC grid 102 including an inductor 554, electrically coupled between thesecond terminals of the switches 530, 534 and one side of the AC grid102 and a capacitor 556 electrically coupled across the components 552.

FIG. 6 shows that embodiments may have the EMI filter 550 comprisingadditional components electrically coupled between the half-bridge 314and the unfolding bridge 316. In such topologies, the inductor 554 maybe electrically coupled between the second terminal of the switch 520and the first terminal of the switch 530 and the capacitor 556 may beelectrically coupled between the first terminals of the switches 530,534.

Each of the electrical switches described herein is a MOSFET in theillustrative embodiments; however, other types of transistors orelectrical switches may be used in other embodiments. In some MOSFETs,the source metallization may connect N and P doped regions on the top ofthe FET structure, forming a diode between the drain and the source ofthe MOSFET, which is represented as body diodes for each of thecorresponding electrical switches. It should be appreciated that, insome embodiments, the inverter 106 may utilize one or more other typesof transistors (e.g., bipolar junction transistors (BJT), insulated-gatebipolar transistors (IGBT), GaN (gallium nitride), etc.) or thyristors.

Turning to FIG. 7, the gate signals used to control the electricalswitches 530, 532, 534, 536 unfolding bridge 316 of the AC-AC converter304 are based on the grid phase angle of the AC grid 102. The controller308 receives the AC voltage V_(grid) and the processor 324 executes analgorithm 700 stored in the memory 326 to estimate the grid phase angleΘ_(grid). FIG. 8A is a simplified electrical schematic diagram of thefour electrical switches 530, 534, 532, 536 (now labeled Q1-Q4,respectively) in the unfolding bridge 316. As illustrated in the plot ofFIG. 8B, representing the grid phase angle Θ_(grid) relative to the gridvoltage V_(grid) as estimated by the algorithm 700, the diagonallyopposite electrical switches Q1, Q4 are turned on and diagonallyopposite electrical switches Q2, Q3 are turned off when the grid voltageV_(grid) has risen above approximately the zero-crossing and is positive(period ‘A’). Conversely, the electrical switches Q1, Q4 are turned offand the electrical switches Q2, Q3 are turned on when the grid voltageV_(grid) has fallen below approximately the zero-crossing and isnegative (period ‘B’). Because the derivation of the grid phase angleΘ_(grid) may be approximate, the control signal may impose a briefblanking time period between the first and second periods during whichall four electrical switches Q1-Q4 are off (period ‘C’). In this way, anegative voltage across the half-bridge 314 may be prevented.

Referring now to FIG. 9, in some embodiments, the inverter controller308 may execute a method 1500 for closed-loop control of the inverter106. The method begins with block 1502 in which the inverter controller308 determines the grid voltage V_(grid) of the AC grid 102 andestimates the angle (θ_(e)) of the grid voltage. As described above, thecontroller 308 may utilize a PLL or other suitable angle estimator to doso. In block 1504, the controller 108 determines the switching frequencyof the electrical switches of the inverter 106 based on the grid voltage(e.g., the instantaneous grid voltage) and/or other circuit parameters.For example, the switching frequency may be determined based on theinput power of the DC source 104, component values of various componentsof the inverter 106 (e.g., component inductances and/or capacitances),and/or operating values of the inverter 106 (e.g., voltages and/orcurrents at various points in the inverter 106). For example, in block1506, the controller 108 may determine the switching frequency based onthe parameters of one or more of the resonant tank circuits of theinverter 106 (e.g., inductance and capacitance values).

In block 1508, the controller 108 determines a phase shift of theactuation signal (θ₁) for the electrical switches of the DC-AC inverter300 relative to the actuation signal (e.g., θ₃=0) for the electricalswitches of the active filter 306. To do so, in block 1510, thecontroller 108 may determine the DC source current for regulation of theDC source voltage of the DC source 104 (e.g., PV panel) based on asuitable MPPT technique. Further, in block 1512, the controller 108 mayeliminate double-frequency ripple from the AC grid 102. In block 1514,the controller 108 may scale the switching frequency for systemlinearized operation as described above. In particular, in block 1516,the controller 108 may scale the actuation signal of the DC-AC inverter300 based on a gain scheduling function of switching frequency. In block1518, the controller 108 may determine a linear operating range forzero-voltage switching based on the DC source voltage and the activefilter voltage.

In block 1520, the controller 108 determines a phase shift of theactuation signal (θ₂) for the electrical switches of the AC-AC converter304 relative to the actuation signal for the electrical switches of theactive filter 306. To do so, in block 1522, the controller 108 maydetermine the grid current of the AC grid 102 for regulation of theactive filter capacitor voltage (e.g., across the energy storage device320) of the active filter 306. Further, in block 1524, the controller108 may eliminate double-frequency ripple from the AC grid 102. In block1526, the controller 108 may scale the switching frequency for systemlinearized operation as described above. In particular, in block 1528,the controller 108 may scale the actuation signal of the AC-AC converter304 based on a gain scheduling function of switching frequency. In block1530, the controller 108 may determine a linear operating range forzero-voltage switching based on the grid voltage and the active filtervoltage. It should be appreciated that, in some embodiments, the blocks1508 and 1520 may be performed in parallel.

In block 1532, the controller 108 generates a set of signals foractuation of the electrical switches of the inverter 106 based on thedetermined phase shifts θ₁ and θ₂. In particular, the controller 108 maygenerate a signal for actuation of the electrical switches of the DC-ACinverter 300, a signal for actuation of the electrical switches of theAC-AC converter 304, and a signal for actuation of the electricalswitches of the active filter 306. The signals may preferably provide50% duty cycles and vary the phase shifts among the ports of theinverter 106 and the switching frequency, thereby controlling the powerflow. These duty cycles may also be set for other percentages, varywithin a margin of error from 50% or other target percentage, and haveduty cycle ranges or targets for both certain instantaneous conditionsand over short or long periods of time for actuating the switches of theDC-AC inverter 300, the switches of the AC-AC converter 304, and theswitches of the active filter 306.

In embodiments the inverter controller 108 or other modules orcomponents may utilize various other techniques to control operations ofinverter 106. For example, in some embodiments, the controller 108 mayutilize an alternative mode of operation for controlling the electricalswitches of the inverter 106. That is, in normal operation, all threeports of the inverter 106 (i.e., the DC-AC inverter 300, the AC-ACconverter 304, and the active filter 306) receive signals to actuate thecorresponding electrical switches of those components. However, duringthe alternative mode of operation, the inverter controller 108 maydisable the signal transmission to one of the ports (i.e., the DC-ACinverter 300, the AC-AC converter 304, or the active filter 306), whichresults in lower switching losses. In particular, in some embodiments,the signal transmission to the port may be disabled every otherswitching period, which may reduce the number of switching instances ofthat port by fifty percent. For example, in some embodiments, thesignals supplied to the active filter 306 may be disabled every otherswitching period.

There is a plurality of advantages of the present disclosure arisingfrom the various features of the apparatuses, circuits, and methodsdescribed herein. It will be noted that alternative embodiments of theapparatuses, circuits, and methods of the present disclosure may notinclude all of the features described yet still benefit from at leastsome of the advantages of such features. Those of ordinary skill in theart may readily devise their own implementations of the apparatuses,circuits, and methods that incorporate one or more of the features ofthe present disclosure and fall within the spirit and scope of thepresent invention as defined by the appended claims.

What is claimed is:
 1. A multi-port inverter for converting an inputdirect current (DC) waveform from a DC source to an output alternatingcurrent (AC) waveform for delivery to an AC grid, the invertercomprising: a transformer; a DC-AC inverter electrically coupled to afirst winding of the transformer, wherein the DC-AC inverter is adaptedto convert the input DC waveform to an AC waveform delivered to thetransformer at the first winding; an AC-AC converter electricallycoupled to a second winding of the transformer, wherein the AC-ACconverter is adapted to convert an AC waveform received at the secondwinding of the transformer to the output AC waveform having a gridfrequency of the AC grid, the AC-AC converter comprising: a first set ofelectrical switches electrically coupled to a first terminal of thesecond winding of the transformer; a capacitor divider electricallycoupled with the first set of electrical switches and to a secondterminal of the second winding of the transformer; a second set ofelectrical switches electrically coupled to the AC grid; a firstcapacitor electrically coupled across the first set of electricalswitches; and a sensor electrically coupled between the first capacitorand the second set of electrical switches, the sensor sensing the ACgrid current; and an active filter coupled to a winding of thetransformer, wherein the active filter is adapted to sink and sourcepower with one or more energy storage devices based on a mismatch inpower between the DC source and the AC grid.
 2. The inverter of claim 1,wherein: the first set of electrical switches comprises a half-bridgecircuit; and the second set of electrical switches comprises anunfolding bridge circuit.
 3. The inverter of claim 2, wherein: thecapacitor divider comprises second and third capacitors having firstterminals electrically coupled at a first node to a first terminal ofthe second winding of the transformer; the half-bridge circuitcomprises: a first electrical switch having a first terminalelectrically coupled at a second node to a second terminal of the secondcapacitor and a second terminal electrically coupled at a third node toa first terminal of a first inductor, the second terminal of theinductor electrically coupled to the second terminal of the secondwinding of the transformer; and a second electrical switch having afirst terminal electrically coupled at a fourth node to a secondterminal of the third capacitor and a second terminal electricallycoupled at the third node to the first terminal of the inductor; thefirst capacitor is electrically coupled between the second and fourthnodes; and the unfolding bridge circuit comprises: a first pair ofseries-coupled electrical switches electrically coupled to each other ata fifth node; and a second pair of series-coupled electrical switcheselectrically coupled to each other at a sixth node, the first pair andsecond pair electrically coupled in parallel at the seventh node and atan eighth node, and the eighth node electrically coupled to the secondnode; a resistor is electrically coupled between the fourth node andseventh nodes, and the fifth and sixth nodes are electrically coupled tofirst and second terminals, respectively, of the AC grid.
 4. Theinverter of claim 1, further comprising an EMI filter having componentselectrically coupled between the unfolding bridge circuit and the ACgrid.
 5. The inverter of claim 1, further comprising an EMI filterhaving components electrically coupled between the half-bridge circuitand the unfolding bridge circuit.
 6. The inverter of claim 1, whereinthe one or more energy storage devices consists of a capacitor andwherein the sensor is a sensing resistor.
 7. The inverter of claim 1,wherein the DC source comprises a photovoltaic module.
 8. The inverterof claim 1, further comprising a controller having a processor and amemory comprising a plurality of instructions stored thereon andexecutable by the processor, wherein: the second set of switchescomprises an unfolding bridge circuit comprising first, second, third,and fourth electrical switches; and in response to execution by theprocessor, the plurality of instructions cause the inverter to controlthe switching cycles of the second set of switches, whereby: when avoltage across the AC grid is substantially positive during a firstperiod, the first and fourth electrical switches are on and the secondand third electrical switches are off; when the voltage across the ACgrid is substantially negative during a second period, the first andfourth electrical switches are off and the second and third electricalswitches are on; and during a third period comprising a blanking timeperiod between the first and second periods when the voltage across theAC grid is approximately zero, the first, second, third, and fourthelectrical switches are off.
 9. A multi-port inverter for converting aninput direct current (DC) waveform from a DC source to an outputalternating current (AC) waveform for delivery to an AC grid, theinverter comprising: an AC-AC converter electrically coupled through atransformer to a DC-AC inverter electrically coupled to the DC source,the AC-AC converter comprising: a half-bridge circuit electricallycoupled to a winding of the transformer; and an unfolding bridge circuitelectrically coupled between the half-bridge circuit and the AC grid;wherein the AC-AC converter is adapted to convert an AC waveformreceived from the transformer to output the AC waveform having a gridfrequency of the AC grid; and a controller having a processor and amemory wherein the controller is adapted to control the switching cyclesof electrical switches of the unfolding bridge circuit, whereby: when avoltage across the AC grid is substantially positive during a firstperiod, a first set of electrical switches is on and a second set ofelectrical switches is off; when the voltage across the AC grid issubstantially negative during a second period, the first set ofelectrical switches is off and the second set of electrical switches ison; and during a third period comprising a blanking time period betweenthe first and second periods when the voltage across the AC grid isapproximately zero, the first and second, sets of electrical switchesare off.
 10. The inverter of claim 9, further comprising an EMI filterhaving components electrically coupled between the unfolding bridgecircuit and the AC grid.
 11. The inverter of claim 9, further comprisingan EMI filter having components electrically coupled between thehalf-bridge circuit and the unfolding bridge circuit.
 12. A multi-portinverter for converting an input direct current (DC) waveform from a DCsource to an output alternating current (AC) waveform for delivery to anAC grid, the inverter comprising: a transformer; a DC-AC inverterelectrically coupled to a first winding of the transformer, wherein theDC-AC inverter is adapted to convert the input DC waveform to an ACwaveform delivered to the transformer at the first winding; an AC-ACconverter electrically coupled to a second winding of the transformerand adapted to convert the AC waveform received at the second winding ofthe transformer to the output AC waveform having a grid frequency of theAC grid, the AC-AC converter comprising: a half-bridge circuitelectrically coupled to the first winding of the transformer; and anunfolding bridge circuit electrically coupled between the half-bridgecircuit and the AC grid; an active filter electrically coupled to awinding of the transformer wherein the active filter is adapted to sinkand source power with one or more energy storage devices based on amismatch in power between the DC source and the AC grid; and acontroller electrically coupled to receive an AC voltage from the ACgrid and having an output signal comprising an estimate of the phaseangle of the AC voltage, wherein the controller, in response to theestimated phase angle of the AC voltage, controls the switching cyclesof a plurality of electrical switches of the AC-AC converter.
 13. Theinverter of claim 12, wherein: the unfolding bridge circuit comprisesfirst, second, third, and fourth electrical switches; and in response tothe estimated phase angle of the AC voltage, the controller controls theswitching cycles of the first, second, third, and fourth electricalswitches, whereby: when a voltage across the AC grid is substantiallypositive during a first period, the first and fourth electrical switchesare on and the second and third electrical switches are off; when thevoltage across the AC grid is substantially negative during a secondperiod, the first and fourth electrical switches are off and the secondand third electrical switches are on; and during a third periodcomprising a blanking time period between the first and second periodswhen the voltage across the AC grid is approximately zero, the first,second, third, and fourth electrical switches are off.
 14. The inverterof claim 12, further comprising an electromagnetic interference (EMI)filter electrically coupled between the unfolding bridge circuit and theAC grid circuit.
 15. The inverter of claim 12, further comprising anelectromagnetic interference (EMI) filter electrically coupled betweenthe half-bridge circuit and the unfolding bridge circuit.
 16. Theinverter of claim 12, wherein the DC source comprises a photovoltaicmodule.
 17. A method for controlling operation of electrical switches ofan unfolding bridge in an inverter configured to convert an input directcurrent (DC) waveform from a DC source to an output alternating current(AC) waveform for delivery to an AC grid, wherein the inverter comprisesa transformer and a DC-AC inverter electrically coupled to thetransformer, an active filter electrically coupled to the transformer,and an AC-AC converter, wherein the AC-AC converter comprises ahalf-bridge circuit electrically coupled to the transformer and anunfolding bridge circuit electrically coupled between the half-bridgecircuit and the AC grid, the method comprising the steps of: determininga phase of the AC grid voltage; and generating a set of signals foractuation of the electrical switches of the unfolding bridge circuitbased on the determined phase, whereby: when a voltage across the ACgrid is positive during a first period, first and fourth electricalswitches are on and second and third electrical switches are off; whenthe voltage across the AC grid is negative during a second period, thefirst and fourth electrical switches are off and the second and thirdelectrical switches are on; and during a third period comprising ablanking time period between the first and second periods when thevoltage across the AC grid is approximately zero, the first, second,third, and fourth electrical switches are off.
 18. The method of claim17, wherein the determining and generating steps are performed in acontroller electrically coupled to the AC-AC converter by one or moreprocessors executing instructions stored in memory.
 19. The method ofclaim 17 further comprising: providing an active filter electricallycoupled to a winding of the transformer wherein the active filter isadapted to sink and source power with one or more energy storage devicesbased on a mismatch in power between the DC source and the AC grid. 20.The method of claim 18 wherein the one or more processors generates theset signals for actuation of the electrical switches based on multipledetermined phase shifts of the AC grid voltage.